The present invention relates to a printing mechanism for a dot matrix impact printer of the crossing type comprising a helical projection provided on the periphery of a platen and a plurality of hammers, each of which hammers is so arranged that the hammer head crosses the helical projection of the platen for performing the dot matrix impact printing.
A dot printer may be classified into an impact printer and a non-impact printer, most of which employ the serial printing system, that is, the printing is serially carried out from end to end of the recording paper. The serial printing system has an advantage that printing cost per unit length on the recording paper is low, if printing width is comparatively long. However, the cost increases with decrease of the printing width.
On the other hand, the impact printer is now largely used, since a clear print may be obtained and, further, printing on sheet paper with carbon copies can be made. Generally, the impact printer employs an inked ribbon printing system, in which printing is performed by impacting a hammer to the recording paper on a drum, with the interposition of the ink ribbon. However the inked ribbon system produces some disadvantages as follows: Construction of the machine is complicated by the ribbon driving means; when a cloth ribbon is used, clearness of the printing is blurred by the texture of the ribbon represented in the print portion; the ribbon must be often changed; the changing operation is troublesome; and the hands of the operator are stained with ink.
If an ink roller inking system comprising an ink roller of sponge material is employed, the above described disadvantages of the ink ribbon system may be eliminated. However, the ink roller inking system has the deficiency that if a portion of the recording paper other than the printed portion accidentally touches a type drum or other parts wetted with ink, the paper is contaminated with the ink. Therefore, in the impact printer, especially in the dot matrix impact printer of the crossing type, it is difficult to design an ink roller inking system which enables printing without the contamination of the paper.